The present invention relates to a plating method for forming a metallic coating on a substrate.
As car accessories such as radiator grilles, back panels, and fog lamp covers having a metallic appearance and provided to automobiles, those manufactured by forming a metallic coating on a substrate are often used. As a method for manufacturing such a car accessory, a plating method is known, in which a conductive coating is formed on a substrate made of a plastic by electroless plating to impart conductivity, followed by forming a plurality of metal-coating layers by electrolytic plating.
FIG. 5 shows a part of the plating step. As a plastic substrate, for example, a substrate made of an acrylonitrile-butadiene-styrene copolymer (ABS) plastic is used. First, the plastic substrate is subjected to a preprocessing step to impart conductivity to the substrate. The preprocessing step includes a degreasing step, an etching step, a catalyst step, an accelerator step, and an electroless nickel plating step.
In the degreasing step, the ABS plastic substrate is subjected to a degrease treatment to remove fats and oils adhered to the surface thereof. In the etching step, the surface of the ABS plastic substrate is roughened (textured) by etching with e.g., chromic acid. In the catalyst step, a catalyst containing a PdSn complex for depositing electroless nickel plating coating is adsorbed to the surface of the ABS plastic substrate. In the accelerator step, the adsorbed catalyst is activated. In the electroless nickel plating step, electroless nickel plating is performed in an electroless nickel plating solution in the presence of a reducing agent containing sodium hypophosphite to form a nickel coating as a conductive coating on the surface of the ABS plastic substrate.
After conductivity is imparted to the plastic substrate by the preprocessing step, the substrate is subjected to an electrolytic plating step in which e.g., a copper plating step, a semi-bright nickel (SBN) plating step, a bright nickel (BN) plating step, a dull nickel (DN) plating step, and a chromium plating step are sequentially applied. A plurality of metallic coating layers is formed in this way on the nickel coating, with the result that not only various functions but also luster metallic appearance are imparted to car accessories.
In the interval between the steps, a plurality of cleaning steps is carried out as necessary to avoid contamination in the subsequent step with an agent(s) used in each step.
In a car accessory manufactured in this way, if it has a complicated shape and recesses in the surface, the thickness of each metallic coating layer formed by electrolytic plating sometimes fails to be uniform. This is because when a metallic coating is formed by electrolytic plating, current density of the inside of a complicated shape and a recess tends to be low, with the result that the thickness of the metallic coating corresponding to these portions becomes extremely thin. Because of this, the whole metallic coating of the car accessory cannot be uniform, with the result that the external shape is not satisfactory as the car accessory.
In the electroplating method described in Japanese Laid-Open Patent Publication No. 2001-073198, it is disclosed that, in order to form a metallic coating being uniform to the inside of an object, electrolytic plating is carried out by arranging an auxiliary electrode in the inside of the object. Owing to use of the auxiliary electrode, the current density at the inside and a recess of the object can be enhanced, with the result that the metallic coating on the inside of the object having the same thickness as that of the metallic coating on the exterior portion of the object can be formed.
However, it is not preferable to apply such an electroplating method in forming a metallic coating on a non-conductive substrate, because metal ions are deposited also on the auxiliary electrode similarly to the non-conductive substrate to form a conductive layer, in the electroless plating performed prior to the electrolytic plating.
Specifically, referring to FIG. 6A, during the electroless nickel plating, a nickel coating 101 is formed as a conductive coating on an ABS plastic substrate 100 by an oxidation-reduction reaction taking place in an electroless nickel plating solution 300 in which a reducing agent containing sodium hypophosphite is present. If preprocessing is applied to an auxiliary electrode 200, which is arranged to conform to the shape of the ABS plastic substrate 100, simultaneously with the ABS plastic substrate 100, in individual steps prior to the electroless nickel plating, the surface of the auxiliary electrode 200 is modified and a nickel coating 201 as a conductive layer is similarly formed.
As shown in FIG. 6B, in copper plating following the electroless nickel plating, the anode 500 immersed in a copper plating solution 400 and an auxiliary electrode 200 are both connected to the anode of a power supply. Then, electric current is applied between the ABS plastic substrate 100 at the cathode of the power supply and the set of the anode 500 and the auxiliary electrode 200. If the auxiliary electrode 200 is positively charged, the nickel coating 201 on the auxiliary electrode 200 is detached and the detached nickel coating 201 sometimes attaches to the surface of the ABS plastic substrate 100 negatively charged. As a result, a copper plating layer 102 is formed that has a projection 202 ascribed to the detached piece and formed on the nickel coating 101 of the ABS plastic substrate 100. In each of the electrolytic plating, i.e., semi-bright nickel (SBN) plating, bright nickel (BN) plating, dull nickel (DN) plating, and chrome plating, a metallic coating is laminated on the projection 202. Because of this, the surface of the resultant car accessory fails to be smooth and its external shape sometimes deteriorates.
The plating method described in Japanese Laid-Open Patent Publication No. 2004-068107 includes a step for forming a uniform coating from the interior to exterior portions of the object without using an auxiliary electrode. According to the plating method, objects are independently placed in cells and the cells are housed in a support communicating with the exterior portion. The objects in the cells are electroplated by rotating the support in a predetermined direction while preventing the cells from falling.
However, in the plating method described in Japanese Laid-Open Patent Publication No. 2004-068107, a rotating mechanism for rotating the support is required. In addition, a large rotational space for arranging a plurality of objects independently in a plurality of cells is required, with the result that the apparatus is enlarged and complicated.